Stellar Occultations By Trans-Neptunian Objects: From Predictions To Observations And Prospects For The Future

In terms of scientific output, the best way to study solar system bodies is sending spacecraft to make in situ measurements or to observe at close distance. Probably, the second best means to learn about important physical properties of solar system objects is through stellar occultations. By combining occultation observations from several sites, size and shape can be derived with kilometric accuracy. Also, atmospheric properties can be derived if the body has an atmosphere. Furthermore, the technique can detect rings and even satellites (although rarely) around the main body. Except for the very special cases of Pluto and Charon, stellar occultations by Trans-Neptunian Objects (TNOs) had never been observed until October 2009. This was because the ephemeris of the TNOs have much larger uncertainties than their angular diameters (typically of the order of ∼10 milliarcsecond) and also because stellar catalogs were not accurate to the milliarcsecond level. Despite the difficulties, at the time of this writing, 43 occultations by 22 different TNOs, and 17 occultations by 5 Centaurs have been detected thanks to the efforts of several teams. Due to the complications of accurately predicting and observing these events, most of the successes have been achieved through wide international collaboration, which is a key issue to succeed in observing stellar occultations by TNOs. Multichord occultations are typically detected at a rate of ∼3 per year on average, whereas the majority of the observed occultations are single-chord detections, which means that only one site detects the occultation. In these cases, no tight constraints on size and shape can be derived from those observations alone. Here we review most of the aspects involved in the complex process to successfully observe occultations, and present some of the lessons learned. There are good prospects for the future if we take advantage of the stellar catalog from the Gaia Data Release 2 (DR2) and if we take advantage of dedicated observational programs of TNOs, to improve their orbits to the required level of accuracy, which is a critical aspect. All this, in combination with large and optimized telescope networks, may allow an increase in the success rate and scientific output of the occultation technique applied to TNOs, in the short-term future.